To calculate BMI manually, divide weight in kg by height in meters squared, or use 703×lb/in² for imperial units.
Body mass index (BMI) is a quick screen of weight relative to height. It uses a short formula, a calculator, and a couple of unit checks. This guide shows How to Calculate BMI Manually in both metric and imperial units, with examples, rounding tips, and common mistakes to avoid. You’ll also see what the result means for adults and where BMI has limits.
How To Calculate BMI Manually (Step-By-Step)
You only need two inputs: weight and height. Use metric if you can; the math is shorter. If you use pounds and inches, one extra factor (703) keeps the scale the same.
Metric Method: Kilograms And Meters
- Measure weight in kilograms.
- Measure height in meters. If you have centimeters, divide by 100.
- Square the height (height × height).
- Divide weight by the squared height.
Formula: BMI = weight(kg) ÷ [height(m)]²
Worked example: 68 kg and 1.70 m.
Height squared = 1.70 × 1.70 = 2.89.
BMI = 68 ÷ 2.89 = 23.5 (rounded to one decimal place).
Imperial Method: Pounds And Inches
- Measure weight in pounds.
- Measure height in inches. If you have feet, multiply feet by 12 and add the extra inches.
- Square the height.
- Multiply weight by 703, then divide by the squared height.
Formula: BMI = 703 × weight(lb) ÷ [height(in)]²
Worked example: 150 lb and 65 in (5 ft 5 in).
Height squared = 65 × 65 = 4,225.
703 × 150 = 105,450.
BMI = 105,450 ÷ 4,225 = 25.0.
Rounding And Units That Keep You Accurate
- Round BMI to one decimal place for readability.
- Use one unit system from start to finish; mixing units is the top mistake.
- Convert cm → m (÷100) and feet→inches (×12) before you square height.
Sample BMI Results For Common Heights And Weights
Use this quick table to see typical BMI values in metric. Your numbers may differ slightly due to rounding. The method is identical across rows: kg ÷ m².
| Height (m) | Weight (kg) | BMI (kg/m²) |
|---|---|---|
| 1.55 | 50 | 20.8 |
| 1.60 | 55 | 21.5 |
| 1.65 | 60 | 22.0 |
| 1.70 | 68 | 23.5 |
| 1.75 | 75 | 24.5 |
| 1.80 | 82 | 25.3 |
| 1.85 | 90 | 26.3 |
| 1.90 | 100 | 27.7 |
Calculating BMI Manually With Pounds And Inches
If you live where pounds and feet are common, the process stays simple with the 703 factor. Convert feet to total inches first, then follow the steps.
Fast Conversions You’ll Use Often
- 1 inch = 2.54 cm
- 1 foot = 12 inches
- To turn feet+inches into inches:
(feet × 12) + inches - 1 kg = 2.20462 lb (round to 2.2 for mental math)
Two More Imperial Walk-Throughs
Example A: 180 lb, 5 ft 10 in (70 in). Height² = 4,900. 703 × 180 = 126,540. BMI = 126,540 ÷ 4,900 = 25.8.
Example B: 130 lb, 5 ft 3 in (63 in). Height² = 3,969. 703 × 130 = 91,390. BMI = 91,390 ÷ 3,969 = 23.0.
What Your BMI Number Means
For adults, BMI groups are standard ranges. They sort weight-for-height into broad screens. Your BMI number is one data point, not a diagnosis.
How The Ranges Work
Public health agencies use these cutoffs in reports and tools. You can read the official categories on the adult BMI ranges page. Below is a compact view for quick reading.
| Category | BMI Range | Notes |
|---|---|---|
| Underweight | < 18.5 | Screen for low weight risk factors. |
| Healthy Weight | 18.5–24.9 | General population target range. |
| Overweight | 25.0–29.9 | Monitor trends and habits. |
| Obesity (Class I) | 30.0–34.9 | Clinical review may be advised. |
| Obesity (Class II) | 35.0–39.9 | Medical care team input may help. |
| Obesity (Class III) | ≥ 40.0 | Specialist pathway often used. |
How To Measure Height And Weight For A Trustworthy Result
Height
- Stand tall with shoes off and back to a wall or stadiometer.
- Heels together, looking straight ahead. Mark the top of the head at a right angle to the wall.
- Record to the nearest 0.5 cm or 0.25 in.
Weight
- Use a scale on a flat, hard surface.
- Weigh at a consistent time of day with light clothing.
- Zero the scale before you step on, then record to the nearest 0.1 kg or 0.2 lb.
Common Calculation Errors (And Easy Fixes)
The Seven Mistakes That Skew BMI
- Squaring the wrong unit. Only square the height number in meters or inches, not cm or feet.
- Mixing systems. If you start in pounds/inches, keep those units through the calculation.
- Dropping the 703. The factor is required with pounds and inches.
- Rounding too early. Keep two decimals through the math; round the final BMI to one decimal.
- Using shoes or thick hair for height. Measure without both.
- Guessing weight. Use a scale; a small guess error changes BMI a lot.
- Copying cm as meters. Convert cm to meters by dividing by 100 first.
When BMI Works Well And When It Doesn’t
BMI is a population screen. It predicts risk patterns at a group level. For one person, extra context matters. Fitness level, fat distribution, age, and sex can shift risk at the same BMI value.
Cases Where BMI Can Mislead
- Very muscular adults. High lean mass raises weight without extra fat; BMI can look high.
- Older adults with muscle loss. BMI may sit in a mid range while fat is higher than it seems.
- Pregnancy. Pre-pregnancy BMI is used for guidance; weight gain targets change during pregnancy.
- Children and teens. BMI is age- and sex-specific in youth; a percentile chart is used.
For a plain summary that puts BMI in context for adults, see the NIH page on calculating BMI. It also reminds readers that BMI is one piece of the puzzle.
Waist Size Adds Clues That BMI Misses
BMI does not capture fat location. Central fat (around the waist) links with higher cardiometabolic risk at the same BMI. A simple tape measure offers extra insight.
How To Measure Your Waist
- Place the tape just above the hip bones, level with the ground.
- Relax the belly. Measure after a normal breath out.
- Record to the nearest 0.5 cm or 0.25 in.
Public health guidance often flags extra risk around >35 in (women) and >40 in (men), used along with BMI and other checks. Clinicians look at the big picture, not one number.
Use These Shortcuts To Double-Check Your Math
Ballpark Rules You Can Do In Your Head
- Metric cue: If height is near 1.70 m, height² is near 2.9. Divide your kg by ~2.9 to spot-check the calculator.
- Imperial cue: Average adult heights range 60–72 in. Height² is then ~3,600–5,200. Multiply your lb by 703 first, then divide by a number in that band. If your BMI looks far off, recheck inches.
- Two-decimal carry: Keep two decimals on height before squaring. Round the final BMI, not the steps.
How To Use Your Result The Smart Way
Your BMI can spark a plan, not label you. Pair the number with daily habits, waist size, and how you feel during activity. Trends over months say more than one reading.
If You Want A Cleaner Baseline
- Repeat the same method next time (same scale, same time of day).
- Log BMI alongside waist size and a short note on activity or sleep.
- Aim for steady, sustainable changes if your trend heads the wrong way.
Printable-Style Walkthrough You Can Reuse
Here’s a compact checklist you can follow any time you need to work out BMI without an app. It keeps metric and imperial steps on one card.
Metric Card
- Weigh yourself in kg.
- Measure height in m.
- Compute height².
- Do kg ÷ height² → record to 1 decimal.
Imperial Card
- Weigh yourself in lb.
- Measure height in inches.
- Compute height².
- Do 703 × lb ÷ height² → record to 1 decimal.
Where To Go Next
If your number sits near a range edge, repeat the measure on another day. If you have health questions, your care team can place BMI in context with labs, history, and goals. You can also cross-check your math with a reputable tool, but now you know How to Calculate BMI Manually without one.